1. Field of the Invention
This invention relates to the field of non-intrusive scanning, and more particularly to systems and methods of determining the average atomic number and mass of a target or one or more portions of a target, as well as to systems and methods of obtaining limits on the mass of certain elements present in a target or in one or more portions of a target.
2. Background Information
A desirable characteristic of a non-intrusive inspection technique for inspecting a target is the ability to rapidly determine atomic number (Z) and density of the target material, as well as the spatial distribution of atomic number and density. In particular, a rapid determination, preferably with low radiation dose, of the distribution in three dimensions of average atomic number and/or mass is a powerful and useful means to determine the contents of a target container. This information may be used to determine a probability that a target container such as a piece of luggage, a shipping container, a storage container, or other container for land, sea, or air transport contains a certain material, such as for example a high-Z and/or a high-density material. Knowledge that a target container contains such a material may be used to identify a threat. For example, the presence of lead in a container may indicate shielding for a “dirty bomb” or other radioactive material. The presence of high Z materials such as uranium may signal the presence of a nuclear weapon in the container. Moreover, measurement of mass distribution, average atomic number, or both may form part of a sequence of inspection techniques. For example, regions of the target that are determined to match specified Z and density categories can be used as input into other inspection techniques that will further probe these regions. Such a system may offer the advantage of providing threat determinations in a convenient time scale.
Techniques for the non-intrusive inspection of cargo include the detection of transmitted radiation (as in x-ray imaging) to obtain a two-dimensional representation of the distribution of density in a target cargo container. Two-dimensional imaging using scattered radiation, such as Compton-scattered radiation, has also been demonstrated. Because of the limitations of two-dimensional imaging, it is often desirable to obtain the density distribution in three dimensions. Non-intrusive scanning to obtain both two- and three-dimensional images of a target using nuclear resonance fluorescence techniques has been described, for example, in U.S. Pat. No. 5,115,459, Explosives Detection Using Resonance Fluorescence of Bremsstrahlung Radiation, and U.S. Pat. No. 5,420,905, Detection of Explosives and Other Materials Using Resonance Fluorescence, Resonance Absorption, and Other Electromagnetic Processes with Bremsstrahlung Radiation, the contents of both of which are hereby incorporated by reference.